The present invention relates to a tube made of vulcanized elastomer comprising polyamide-based and EVOH barrier layers.
The invention is useful for fluids in air conditioning systems. It is also useful for liquids containing volatile substances, preventing the liquid from being depleted in this volatile substance. The invention is also useful for the cooling liquid in engines and for oil. The tubes of the invention are, for example, of the type:
rubber (outer layer)/EVOH/blend of polyamide and of polyolefin having a polyamide matrix.
Patent EP 683 725 describes hoses consisting, in succession, of an inner layer made of PVDF (polyvinylidene fluoride), a coextrusion tie and an outer layer made of vulcanized elastomer. They have the advantage of exhibiting good resistance to aggressive chemical fluids and of being a barrier to many fluids, particularly petrol and the fluids used in air conditioning circuits. However, they may be brittle at low temperature. It is known to improve the impact strength of PVDF, but this is to the detriment of its chemical resistance and its barrier properties.
The prior art has described many tubes having a polyamide outer layer for motor-vehicle petrol. Thus, Patent Application EP 0 731 308 discloses tubes based on polyamides and on EVOH for transporting petrol. These tubes may have a four-layer structure comprising, respectively, a PA-12 outer layer, a binder layer, which is a grafted polyolefin, an EVOH layer and an inner layer in contact with the petrol, comprising a blend of a polyamide and a polyolefin having a polyamide matrix.
Patent EP 428833 describes a three-layer tube comprising, respectively, a PA-12 outer layer, a binder layer which is a grafted polyolefin and an EVOH inner layer in contact with the petrol.
Patents EP 428834 and EP 477606 describe a five-layer tube comprising, respectively, a PA-12 outer layer, a binder layer which is a grafted polyolefin, a PA-6 layer, an EVOH layer and a PA-6 inner layer in contact with the petrol.
U.S. Pat. No. 5,038,833 describes a three-layer tube comprising, respectively, a PA-12 outer layer, an EVOH layer and a PA-12 inner layer in contact with the petrol.
Patent EP 1 036 967 describes a polyamide-based multilayer tube, characterized in that it comprises, in its radial direction from the inside outwards:
an inner layer formed from a polyamide or from a blend of a polyamide and a polyolefin with a polyamide matrix, this layer including a dispersed filler consisting of electrically conducting carbon black producing a surface resistivity of less than 106 xcexa9/xe2x96xa1;
an interlayer formed from a polyamide or from a blend of a polyamide and a polyolefin having a polyamide matrix, this layer not having any electrically conducting carbon black or an electrically significant amount of this carbon black;
an EVOH layer;
a tie layer;
a polyamide outer layer,
the above layers adhering to one another in their respective contact region. This tube has very good mechanical properties and is impermeable to petrol.
All these tubes having a polyamide outer layer are too expensive for air conditioning fluids. This is because such tubes have been designed to withstand very severe mechanical conditions, in terms of the inflammability of petrol and this mechanical behaviour is not necessary for air conditioning fluids. In addition, these polyamide-based tubes are too rigid and it is difficult to fit them into motor vehicles. They are suitable for petrol because an outside diameter of 8 mm is sufficient, but for air conditioning the diameters may be larger.
What has now been developed is a tube made of a vulcanized elastomer (rubber) comprising an EVOH layer and a layer of a blend of a polyamide and a polyolefin having a polyamide matrix.
The present invention relates to a multilayer tube comprising, in its radial direction from the outside inwards:
a first layer of vulcanized elastomer forming the outer layer,
a second layer of EVOH or of an EVOH-based blend,
a third layer of a blend of a polyamide (A) and a polyolefin (B) having a polyamide matrix,
optionally, an inner layer of vulcanized elastomer,
the layers being successive and adhering to one another in their respective contact region.
It is also possible to place a tie layer between the first and second layers, and likewise between the optional inner layer and the third layer.
The tube of the invention may also include a reinforcing layer of the textile type, for example made of polyester or of metal wires, the said layer being placed between the first and second layers. This reinforcing layer may be between the tie layer and the EVOH layer or between the tie layer and the outer layer or else the tie may be placed in the interstices of the reinforcing layer.
The tubes of the invention may have an outside diameter of between 8 mm and 25 cm. The thickness of the EVOH layer may be between 10 and 200 xcexcm, that of the blend of the polyamide (A) and the polyolefin (B) having a polyamide matrix between 25 and 500 xcexcm, and that of the optional tie between 10 and 100 xcexcm.
These tubes can be manufactured by coextrusionxe2x80x94each layer is introduced in the melt using an extruder into a coextrusion head which produces concentric streams forming the tube. This technique is known per se. The tube is then passed through an oven or heating tunnel in order to carry out the vulcanization (crosslinking) of the elastomer. It is recommended during the coextrusion to use a coextrusion head in which the stream of elastomer remains at a sufficiently low temperature (in general about 80 to 120xc2x0 C.) in order not to cause vulcanization before the tube has been formed and above all to prevent the extruder from becoming blocked. It is also possible to manufacture by coextrusion a tube which does not include the first elastomer layer, and then to make this tube pass through a device called a xe2x80x9cjacketingxe2x80x9d device or xe2x80x9ccrossheadxe2x80x9d in order to cover it with the elastomer layer. All that is then required, as above, is to pass the tube through an oven or heating tunnel in order to vulcanize (crosslink) the elastomer. If the tube includes an inner layer of vulcanized elastomer, the process then starts with the manufacture of a tube consisting of just this layer, which is then vulcanized and put back through a device called a xe2x80x9cjacketingxe2x80x9d or xe2x80x9ccrossheadxe2x80x9d device in order to cover it with all the other layers, apart from the outer elastomer layer, the process then continuing as above.